Method and device for recording marks representing data in an information layer of an optical record carrier

ABSTRACT

The present invention relates to a method of recording marks representing data in an information layer of a record carrier by irradiating the information layer by means of a pulsed radiation beam, each mark being written by a sequence of pulses, the recorded marks being erasable by irradiating the information layer with an erase radiation beam. In order to achieve a reduction of the temperature of a transparent heat sink of a dual-layer optical recording medium without compromising the direct overwrite of old data, it is proposed according to the present invention that said erase radiation beam has a first erase power level (e 1 ) for a first erase period (t 1 ), a second erase power level (e 2 ) higher than or equal to said first erase power level (e 1 ) for a second erase period (t 2 ), and a third erase power level (e 3 ) lower than said second erase power level (e 2 ) for a third erase period (t 3 ).

The present invention relates to a method of recording marksrepresenting data in an information layer of a record carrier byirradiating the information layer by means of a pulsed radiation beam,each mark being written by a sequence of pulses, the recorded marksbeing erasable by irradiating the information layer with an eraseradiation beam. The invention further relates to a corresponding opticalrecording device for recording marks representing data in an informationlayer of a record carrier by irradiating the information layer by meansof a pulsed radiation beam, the device comprising a radiation sourceproviding said radiation beam and a control unit for controlling thepower of said radiation beam such that each mark is written by asequence of pulses, the recorded marks being erasable by irradiating theinformation layer with an erase radiation beam.

Such a method and device are known from the WO01/04885 A1. Therein amark is written in a phase change layer of a record carrier by asequence of radiation pulses. A trailing power level having a valuehigher than the erase power level is introduced after a last write pulsein a sequence of radiation pulses. Additionally, the power level of thelast write pulse in a sequence may be raised. This results in a reducedjitter of the marks being written, especially when writing takes placeat high recording speeds. In this way marks are written in a phasechange layer of a record carrier. The area between two successive marksis referred to as a space. The pattern of marks and spaces on a recordcarrier represents the information stored on that record carrier.

A factor-of-two storage capacity increase of a rewritable opticalrecording media can be achieved by introduction of a second recordinglayer such as described in “Title Phase-Change Recording: Options for 10to 20 GB (dual layer, high NA, and blue)”, Wierenga H. A., Proceedingsof the SPIE The International Society for Optical Engineering (USA),vol. 3401, p. 64-70, 1998. To access both recording layers, the layercloser to the objective lens of the recording device needs to betransparent. Making such a transparent recording layer requires a majorchange of the recording layer stack-design. In single layer discs, thisstack consists typically of a metal mirror layer, dielectricinterference layers, and a phase-change layer. However, these types ofstacks are not transparent due to the metal mirror layer. Leaving out ofthe metal mirror layer in dual-layer discs has not only consequences forthe behavior of the stack (that is becoming at least partiallytransparent), but also for its thermal characteristics.

In transparent dual-layer recording stacks, transparent heat sinks areintroduced to achieve at least a moderate cooling of the phase-changelayer without compromising the transparency of the stack. The slowercooling characteristic of such a stack results in a heat accumulationwhich causes two kinds of problems. First, longer cooling gaps inbetween the write pulses are needed during writing of marks. This can beachieved by using shorter write pulses. Second, the temperature of theheat sink needs to be low at the beginning of a mark. The moststraightforward solutions are a reduction of the erase power during thespace write-strategy or the introduction of a cooling gap before thefirst write pulse. However, experiments in optical recording systems(such as, for example, DVD) have shown that both strategies result in apoor overwrite performance.

It is therefore an object of the present invention to provide a methodand a device for recording marks in an information layer of a recordcarrier by which a reduction of the temperature of a transparent heatsink is accomplished without compromising the overwrite performance.

This object is achieved according to the present invention by providinga method according to the preamble wherein said erase radiation beam hasa first erase power level for a first erase period, a second erase powerlevel higher than or equal to said first erase power level for a seconderase period, and a third erase power level lower than said second powerlevel for a third erase period. The object is further achieved byproviding a corresponding recording device as claimed in claim 11.

The present invention is based on the idea to divide the space writestrategy into at least two, preferably three, different power levels. Atthe beginning of a space, a one- or two-stage erase power level boost isused to heat the stack quickly. The initial stage of a one-stage boostor the first stage in case of a two-stage boost is needed to preventre-crystallization at the end of the previous mark. Thereafter, that isafter said initial stage or after the second boost of a two-stage boost,the erase power is lowered to a value which is substantially lower thanthat required in conventional space write strategies without a boost.The net result is the desired reduction of the stack temperature beforethe following mark to be recorded. The combination of the abovedescribed new write strategy and a stack with one transparent heat sinklayer, preferably made of Indium Tin Oxyde (ITO), especially results ina very good overwrite performance.

Embodiments of the invention are defined in the dependent claims.Advantageous embodiments for the different power levels and thedifferent erase periods used in the space write strategy are defined inclaims 2 to 7. Preferably, the third erase power level is lower than theerase power level applied in conventional space write strategies, whileat least the second erase power level is higher than the conventionalerase power level.

In an embodiment of the invention, the first and third erase powerlevels are substantially equal and lower than the second erase powerlevel.

Furthermore, the sum of the first and second erase periods is preferablyshorter than half the shortest effect (i.e. mark) length, for example a1T duration for a code with 2T as the shortest effect (i.e. mark)length.

The space write strategy according to the present invention using atwo-stage erase boost is preferably applied to direct overwrite (DOW) ina (semi-) transparent layer of an optical record carrier comprising atleast two information layers. Write strategies using such an erase boostcould, for example, be applied in DVD and DVR dual-layer recording.

The information layer is preferably of the type which has a phase whichis reversibly changeable between a crystal phase and an amorphous phase.Because of the similarity of the cooling problems in dual-layer recordcarriers and in high speed recording of single layer record carriers,the method and recording device according to the present invention, inwhich the above described erase power level boost is applied, could alsobe advantageously applied to high-speed CD-RW, DVD, and DVR recording.

The invention will now be explained in more detail hereinafter on thebasis of preferred embodiments and with reference to the accompanyingdrawings, in which

FIG. 1 a shows a cross-section of a dual-layer record carrier,

FIG. 1 b shows a cross-section of a semi-transparent information layer,

FIG. 2 shows diagrams illustrating a data signal and a correspondingcontrol signal for controlling the power levels of the radiation beamaccording to an embodiment of the present invention having a two-stageboost,

FIG. 3 shows results of measurements of the single-track jitterillustrating the advantageous effect of the invention,

FIG. 4 is a block diagram of a recording device according to theinvention, and

FIG. 5 shows diagrams illustrating control signals according toembodiments of the present invention having an one-stage boost.

FIG. 1 a is a schematic representation of a dual-layer disc-shapedrecord carrier 1. The disc 1 comprises a polycarbonate substrate 10, tworecording layers of the phase-change type 11, 13 separated by a spacerlayer 12, and a cover layer 14. Data recorded in the information layers11, 13 are written or read by a radiation beam 2, such as a laser lightbeam. If data are to be read from or written into the information layer11, the other information layer 13 closer to the surface facing thelaser beam 2 should be, at least partially transparent.

FIG. 1 b shows an example of a cross-section of such a semi-transparentinformation layer 13. As shown, it comprises a phase-change layer 132and a transparent heat sink layer 134 which are separated and covered bydielectric layers 131, 133, 135. Other stack arrangements arealternatively possible.

FIG. 2 shows diagrams of two signals used in an embodiment of thepresent invention, a digital data signal 20 and a control signal 30.FIG. 2 a gives the value of the digital data signal 20 as a function oftime, the value of the signal representing information to be recorded.When recording this data signal 20, a “high” period 21 is recorded as amark having a length corresponding to the duration of the “high” period.A “low” period 22 is recorded as a blank area, a space, situated betweenthe marks and having a length corresponding to the duration of the “low”period. In general, the length of a mark is substantially equal to thenumber of channel bit periods (number of periods of the data clock) ofthe data signal times the writing speed.

The data is written in an optical record carrier 1 having one or moreinformation layers, such as an optical record carrier having twoinformation layers as shown in FIG. 1. The marks representing the dataare written along a track in the information layer by irradiating theinformation layer with a pulsed radiation beam. The marks are areas ofthe information layer having optical characteristics different fromtheir surroundings, thus rendering an optical reading of marks possible.

FIG. 2 b shows a control signal 30 corresponding to the data signal 20in an embodiment of the invention. The control signal 30 is used formodulating the power of a radiation beam with which the marks arewritten on the information layer, where it is assumed that the powerlevel of the radiation beam is proportional to the level of the controlsignal. FIG. 2 b shows, as a function of time, a last pulse 31 of asequence of write pulses for writing a mark 21. These write pulses havea write power level w, while the power level between the write pulses ina sequence is a bias power level b. Furthermore, a first pulse 32 of asequence of write pulses for writing the next mark 21 after the space 22is shown on the right-hand side. Previously written marks, if any,between the marks 21 being written are erased by means of an erasewrite-strategy according to a preferred embodiment of the presentinvention.

At the beginning of the space 22, a two-stage boost is used to heat thestack quickly. The low-level first stage 33 having a first erase powerlevel e1 for a first erase period t1 is used to preventre-crystallization at the end of the previous mark 21. Thereafter, asecond boost 34 is applied having a second erase power level e2 higherthan the first erase power level e1 for a second erase period t2. Afterthe second boost 34, the erase power is lowered to a third erase powerlevel e3 lower than the first erase power level e1 for a third eraseperiod t3, that is, for the remaining time of the space 22 (referencenumeral 35).

Also shown as a dashed line in this diagram is a conventional spacewrite-strategy without erase boost according to the invention. In such aconventional space write strategy, the erase power level is set to aconstant level e0 higher than the third erase power level e3 during thewhole period for writing the space 22. It should be noted that also aone-stage boost, where e1 is equal to e2, may be used.

The net result of the space write strategy according to the invention isthe desired reduction of the stack temperature. The combination of thisnew write strategy and a stack with one transparent heat sink layer asshown in FIG. 1 results in a very good overwrite performance, as can beseen from FIG. 3. Here the average single-track (ST) jitter is shown asa function of the number of direct overwrite (DOW) cycles. Aconventional write strategy with a cooling gap at the end of the markshows a poor overwrite performance (line 41 with diamonds), whereasboosting of the erase power level at the begining of a space togetherwith a reduced erase power e3 results in a good overwrite performance(line 42 with squares), as is illustrated in FIG. 2 b

It should be noted that the optinum values of the erase power levels e1,e2, e3 as well as the erase periods t1, t2, t3 depend on the propertiesof the recording medium, the laser pulse, and the laser spot on theinformation layer. As an example, the following values could be used:t1=½T; t2=½T; e1=2.3 mW; e2=2.6 mW; e3=1.1 mW.

It should be noted that, instead of the two-stage boost as shown in FIG.2 b, a one-stage boost may alternatively be used. An embodiment of suchan one-stage boost is illustrated in FIG. 5 a where the first and seconderase power levels e1 and e2, which together form the one-stage boost,are equal but higher than the third erase power level e3. The first andsecond erase power levels e1 and e2 of the boost are higher than theconstant erase power level e0 used in a conventional space writestrategy. A further embodiment is illustrated in FIG. 5 b where thefirst and third erase power levels e1 and e3 are equal, but lower thanthe second erase power level e2 forming the one-stage boost. Again, thesecond erase power levels e2 of the boost is higher than the constanterase power level e0 used in a conventional space write strategy.

It should be noted that the term “equal” does not necessarily mean“exactly the same”, but rather “approximately equal”, since slightdifferences from the equal level do not take the advantages of theinvention away.

FIG. 4 shows an embodiment of a recording device according to thepresent invention for recording data on a disc-shaped optical recordcarrier 1. Alternatively, the record carrier may be in the form of atape. A data signal S_(D), comprising the information to be recorded, isapplied to a control unit 60. A current source 61 within the controlunit 60 has five outputs, A, B, C, D, and E. Output A provides a currentwhich, when fed into a radiation source 51 through a control signalS_(C), will result in a radiation beam 52 having a write power level(w). Likewise, outputs B, C, D and E provide currents resulting in thebias power level (b), the first erase power level (e1), the second erasepower level (e2), and the third erase power level (e3), respectively.The current of each output A, B, C, D, and E can be selected by a switchunit 62. The switch unit 62 is operated by a pattern generator 63controlled by the data signal S_(D) and a clock signal S_(K). Thepattern generator 63 transforms the data signal S_(D) into the controlsignal having the power levels in accordance with a desired pattern. Theclock signal S_(K) is obtained from a clock generator 58.

The control signal S_(C) provided at the output of the control unit 60is applied to the radiation source 51 and controls the power of theradiation beam 52 generated by the radiation source 51. The radiationbeam is focussed onto an information layer 11 (or 13) of the recordcarrier 1 by a lens 53. The disc-shaped record carrier 1 is rotatedabout its center by a motor 54.

1. A method of recording marks representing data in an information layerof a record carrier by irradiating the information layer by means of apulsed radiation beam, each mark being written by a sequence of pulses,the recorded marks being erasable by irradiating the information layerwith an erase radiation beam, characterized in that said erase radiationbeam has a first erase power level for a first erase period, a seconderase power level higher than or equal to said first erase power levelfor a second erase period, and a third erase power level lower than saidsecond erase power level for a third erase period.
 2. A method asclaimed in claim 1, wherein said third erase power level is lower thansaid first erase power level.
 3. A method as claimed in claim 1, whereinsaid first erase power level and said third erase power level aresubstantially equal and lower than said second erase power level.
 4. Amethod as claimed in claim 1, wherein said second erase power level islower than the write power level (w) of said pulses of said pulsedradiation beam for recording marks.
 5. A method as claimed in claim 1,wherein said third erase power level is higher than the bias power level(b) between said pulses of said pulsed radiation beam for recordingmarks.
 6. A method as claimed in claim 1, wherein said first eraseperiod and said second erase period are shorter than said third eraseperiod.
 7. A method as claimed in claim 1, wherein the sum of said firsterase period and said second erase period is shorter than half theshortest mark being recorded.
 8. A method as claimed in claim 1, whereinsaid information layer has a phase which is reversibly changeablebetween a crystal phase and an amorphous phase.
 9. A method as claimedin claim 1, wherein said record carrier comprises at least twoinformation layers.
 10. A method as claimed in claim 9, wherein at leastone of said at least two information layers is an at least partiallytransparent layer.
 11. A recording device for recording marksrepresenting data in an information layer of a record carrier byirradiating the information layer by means of a pulsed radiation beam,the device comprising a radiation source providing said radiation beamand a control unit for controlling the power of said radiation beam,such that each mark is written by a sequence of pulses and the recordedmarks are erasable by irradiating the information layer with an eraseradiation beam, characterized in that a control unit is operative forcontrolling said radiation beam such that said erase radiation beam hasa first erase power level for a first erase period a second erase powerlevel higher than or equal to said first erase power level for a seconderase period, and a third erase power level lower than said second erasepower level for a third erase period.
 12. A recording device as claimedin claim 11, wherein said control unit is operative for controlling saidradiation beam such that the third erase power level is lower than thefirst erase power level.
 13. A recording device as claimed in claim 11,wherein said control unit is operative for controlling said radiationbeam such that the first erase power level and the third erase powerlevel are substantially equal and lower than the second erase powerlevel.